1. Field of the Invention
This invention relates to a method and apparatus of electrodeposition coating of vehicle bodies or the like, and more specifically to a method and apparatus of multistage power conduction in cationic electrodeposition coating.
2. Description of the Prior Art
According to the conventional multi-stage power conduction method which has been employed in the electrodeposition coating, a power supply including an AC power transformer and a rectifier is independently provided for each conduction stage. In conduction stages which precede the final conduction stage located at the end in the direction of transfer of the work to be coated, a low voltage or a of gradually increasing voltage is applied for the purpose of preventing defects in the quality of the electrodeposited film, while a high voltage is applied in the final conduction stage at the end of transfer of the work in order to ensure a good coating of desired thickness of the electrodeposited film and a thorough electrodeposition thereof. However, the application of high voltage in the final conduction stage is normally feasible only for a short time due to limitation of the length and size of the electrodeposition bath, often resulting in degrading the coating in terms of thickness of the electrodeposition film and thorough electrodeposition.
On the other hand, if high voltage is applied in the first conduction stage in a single- or multi-stage conduction method for the purpose of ensuring the thickness and adhesion of the electrodeposited film and thorough electrodeposition, there occur rashes, pinholes or irregular steps on the coated film surface especially in the case of cationic electrodeposition coating, thereby failing to attain a desirable quality of the coating.